Naturally occurring RNA-containing sarcoma viruses, that transform fibroblasts in tissue culture, have been isolated from a number of mammalian species. Investigations are being performed to characterize new sarcoma virus isolates with respect to their biological properties and gene products. The properties of sarcoma virus-induced transformants are being compared with tissue culture lines derived from naturally occurring malignancies of human origin to ascertain whether analogies can be found in the mechanisms of neoplastic change.